The present invention relates generally to an improved assembly comprising a combination of a thermally conductive electrically insulative multi-layered laminate with one of the outer layers of the laminate having a printed circuit or a surface mounting arrangement for receiving and mountably positioning a semiconductor device thereon. The assemblies fabricated in accordance with the present invention are particularly adapted for use as highly responsive thermally conductive mounting pads forming an electrical isolation barrier with the chassis when one or more solid-state electronic devices are operatively mounted on the pad and electrically coupled to the circuitry. The arrangement utilizes a plurality of electrically insulative and thermally conductive layers interposed between two or more highly thermally conductive layers. One of the highly thermally conductive layers serves as the ultimate strata or rigid base layer upon which the other layers are mounted, and as such is properly termed the base or substrate for the assembly. The outer layer forming the opposed surface normally serves as a circuitry array and/or surface mounting pad or arrangement for one or more solid-state devices. In order to improve the electrical properties for the assembly, it is normally preferred that a plurality of relatively thin electrically insulative layers be employed. The thermally conductive properties for the assembly may be improved by interposing a highly thermally conductive layer of reasonable thickness between electrical isolation layers, thereby providing a means for rapidly spreading thermally energy being transferred across and through the individual electrical isolation barriers. Thin layers of epoxy resin adhesive, preferably in multiple layer form, are typically employed to bond together the individual layers of the assemblies of the present invention.
The entire assembly is normally mounted upon the chassis which forms the ultimate support and heat sink, normally providing a support for one or more thermally conductive substrate assemblies, and with each such assembly having one or more semiconductor devices disposed thereon. The thermal and electrical properties required are that each of the layers comprising the laminated structure be thermally conductive and with at least one intermediate layer of the laminate being electrically insulative and with certain others being electrically conductive. The individual layers are securely bonded together as an assembly, with air entrapment and/or voids being, of course, carefully avoided. Multiple layers of electrically insulative particulate solid laden film are preferably employed, and it is frequently desirable to interpose a thermally conductive metallic layer between such insulative layers for heat spreading purposes. The interposed metallic layer effectively increases and/or otherwise expands the effective area for thermal conductivity through the laminated structure, with this interposed metallic layer thereby functioning primarily as a thermal energy distributing or heat spreader layer for the overall assembly. The rate of heat dissipation for the semiconductor device mounted thereon is accordingly increased.
One outer layer of the thermally conductive electrically insulative laminate, as indicated above, serves as a surface mounting arrangement for one or more solid-state devices and may be in the form of a printed circuitry layer appropriately coupled to the solid-state device. This layer is preferably fabricated from a material such as copper, or aluminum-clad copper. In certain applications, solder films or layers may be provided on the surface of the copper. Such assemblies do, of course, contribute to ease of permanent mounting of solid-state devices thereon.
The combination of properties provided in the present arrangement is not available in nature. Accordingly, the present invention which provides these unusual results comprises a multi-layer laminate incorporating a metallic base or substrate, with one or more distinct electrically insulative and thermally conductive layers being disposed thereon, and with one or more conductive metallic layers frequently being interposed between the insulative layers, including one printed circuitry layer or surface mounting layer being arranged on the outer surface. A solid-state electronic device is normally mounted on the surface of the printed circuitry, and additionally, other devices such as surface mount resistors and/or leadless chip carriers including one or more chip assemblies may be received on the surface of the outer (upper) circuitry layer. The subject matter of the present invention is related and constitutes an improvement to that disclosed in co-pending applications Ser. No. 584,897, filed Feb. 29, 1984, entitled "MOUNTING PAD FOR SOLID-STATE DEVICES", now U.S. Pat. No. 4,574,879, and Ser. No. 752,669, filed July 8, 1985, entitled "LAMINATED MOUNTING PAD FOR SOLID-STATE DEVICES", both applications being assigned to the same assignee as the present invention.
Materials, including elements, compounds, and compositions of matter rarely possess the combined properties of being both highly thermally conductive and highly electrically insulative. Since the materials possessing such a combination of properties are rare, one must ordinarily seek compromises in one or more of the physical and electrical properties in order to find a useful material or combination of materials. The efficacy of the final products accordingly become limited.
One technique for decreasing the thermal impedance in an electrically insulative material is to utilize such a material in a form having a thin cross-sectional thickness. The thickness is, however, consistent with the required mechanical and electrical properties. However, as the cross-sectional thickness decreases, the risk of rupture, cracking, or fracture of the insulative material increases, thereby increasing the risk for reduction or derogation of electrical properties. Such derogation may lead to an increased risk of electrical failures for devices functionally mounted thereon. In order to further decrease the thermal impedance in an electrically insulative film, it is now possible to load such films with a quantity of particulate solid material having reasonable thermal conductivity. In the devices of the present invention, polyimide(amide) or epoxy films loaded with particulate solids are employed.
In the present arrangement, it has been found desirable to utilize multiple thin layers or films of electrically insulative materials in order to enhance the electrical properties of the overall assembly without unusual sacrifice of the physical properties of the assembly. Typically, electrically insulative films or layers are filled with a thermally conductive material in the form of a particulate solid. In one aspect of the present invention, multiple thin layers of electrically insulative material may be employed, with highly thermally conductive metallic layers of reasonable cross-sectional thickness being interposed therebetween. In such a structure, therefore, the thermal properties of individual multiple thin layers normally function on a synergistic level or basis rather than merely on an additive or accumulative basis. The intervening metallic layers function to increase the effective area of thermal conductivity through the assembly, while at the same time increasing the mechanical stability and durability of the entire assembly. The insulative layers are normally somewhat flexible or pliable, and as such may function as energy absorption layers when the assembly is subject to heat stresses such as those experienced during normal production operations. Thus, when subjected to those heat or temperature excursions normally experienced in component or die mounting processes, delamination or other problems which occasionally arise at the interfaces between layers are virtually eliminated.
Further desirable properties or characteristics in the layers or combinations of layers comprising the finished product include chemical resistance, mechanical toughness, as well as mechanical durability. In order to obtain this combination of properties, the insulating member or members and the circuitry pattern must be resistant to chemicals encountered in printed circuit processes, as well as resistant to failures which may be caused by such mechanical operations as cutting, molding, broaching, coining or folding, which may result in cutting, ripping, cracking, or puncturing of certain of the layers. The mechanical and electrical properties as set forth hereinabove are desirable in order to provide an electrical mounting member which is sufficiently tough and durable so as to withstand the thermal energy and peak temperatures reasonably expected to be reached during subsequent assembly processes and also during its subsequent functional operation. For example, the mounting member frequently must withstand the rigors of exposure to those harsh chemicals normally encountered during conventional printed circuit fabrication operations. The finished product must further provide a durable mounting arrangement to permit application of and withstand utilization of conventional final attachment or mounting techniques including the use of mounting screws and other conventional component-mounting devices.
With respect to other physical, thermal and electrical properties which are desirable for use in combination with high power type solid-state devices, it is desirable that the laminate structure as well as individual layers in the laminate possess thermal properties that allow exposure of the assembly to temperature excursions experienced during such processing operations as soldering, brazing and welding. These processes and operations can include exposure to temperatures up to about 400.degree. C., sometimes undertaken in inert or hydrogen atmospheres. Typically, soldering operations may include exposure to temperatures in the area of about 200.degree. C., with certain brazing operations including exposure to temperatures in excess of about 425.degree. C. Materials fabricated from or including copper cannot be expected to possess normal surface characteristics after exposure to temperatures exceeding about 260.degree. C., particularly if such temperature levels are reached while in ordinary atmospheres of air. This limitation on copper is due to the formation of oxides of copper on the surface, with the resultant modification of surface characteristics and, in some instances, loss of strength. A coating or plating of nickel can assist in reducing the formation of oxides of copper, and such coating or plating operations is normally advantageous when dealing with the substance of the present invention.
Exposure to temperatures in the range set forth above are discussed here in order to provide an indication of the variety of conditions to which devices of the present invention may be exposed even though on a short-term basis. While temperature excursions in the area of about 350.degree. C. or higher may be a relatively short duration, such as during certain high temperature soldering operations, the materials utilized must permit the solid-state device and the assembly components to retain their physical and electrical integrity during and after such epposure. Processing temperatures in the area of 350.degree. C. are not unusual, and are typically encountered when conventional 95:5 solder alloys are being utilized. Such solder alloys are frequently employed in the initial soldering operations in certain subassemblies, with later operations employing soldering alloys having somewhat lower melting temperatures. Because of the overall or combined thermal properties of the components comprising the laminates of the present invention, damage to the mounting assembly will not be likely to occur when these normal elevated temperature excursions occur during either processing operations or actual operation of the semiconductor component. Furthermore, because of the demands of production processes, both solvent and chemical resistant properties of the materials are desirable, and the devices of the present invention possess these properties.
Because of their high temperature performance characteristics, polyimide(amide) films have been utilized in combination with semiconductor devices, such as diodes, transistors, integrated circuit devices and the like. In connection with the present invention, filled polyimide(amide) films are utilized in combination with other layers having specific properties in order to provide a highly durable and practical mounting apparatus. For enhanced performance, the polyimide(amide) film in the arrangement of the present invention contains a quantity of a particulate solid, such as from between about 5% and 65% by volume, normally about 20%-50% weight, of a material such as alumina or boron nitride, with the particulate solids having an average particle size along the major dimension ranging from between about 2 microns to 30 microns. These particulate solids are incorporated directly into the polymer matrix, and polyimide(amide) films having such materials incorporated therein are commercially available from E. I. DuPont deNemours Corp. of Wilmington, Del. under the trade name "Kapton M. T." The preparation of the polyimide(amide) base films is described in U.S. Pat. Nos. 2,149,286; 2,407,896; 2,421,024; and 2,502,576. In order to preserve the desirable electrical and physical properties of the polyimide(amide) films, the incorporation of solids is undertaken so as to insure reduction of the quantity of entrained gases such as air in the film. Uniform imperforate films of polyimide(amide) resin are available, with film thicknesses as low as one-quarter mil having been found useful in connection with the assemblies of the present invention. In addition, such films having a thickness of up to about 5 mils are particularly useful, and films of greater cross-sectional thickness may also be utilized.
A further advantage available from the use of polyimide(amide) films loaded with particulate solids is that the high temperature capability of the films make it possible to employ certain metallic treatment techniques which are not normally undertaken when conventional base films are used. Such unusual metal treatment techniques include thermal compression, laser bonding of metals, or flip-and-press bonding operations.
In the past, attempts have been made to utilize thin films in combination with adhesive layers to form a mounting structure. However, such devices have not employed the combination of a polyimide(amide) film loaded with particulate solid, together with a heat spreader layer of the type employed in connection with the present invention. The combination of features available from the components utilized in connection with the present invention provide a finished product with enhanced operational characteristics.